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Linux.com: CLI for Noobies: test This

"Last week I mentioned in closing that there would be a test. I
hope you took that seriously, you malingering, mousiefied,
GUI-addicts, because this week is test week. So brace yourself for
a blast of fresh air and open a console window, we're going into
command-line territory.

"The test command allows you to conditionally perform some task
based on the existence, type, or status of a file. It also lets you
do the same thing by comparing integer and string values. For my
sake, we'll keep it as simple as possible and stick to file tests
while still showing off some of its potential..."